A thin film transistor (TFT) is a switching device in which a polysilicon thin film is used as an active layer. In general, the thin film transistor is used as an active device of an active matrix liquid crystal display apparatus, a switching device of an electro-luminescence display apparatus, and peripheral devices thereof.
The thin film transistor is typically manufactured by using a direct deposition process, a high temperature process, or a laser thermal treatment process. By using the laser thermal treatment process, a crystallization (or a phase transition) can be obtained even at a lower temperature of 400° C. or less, and high field effect mobility can be implemented. Therefore, the laser thermal treatment process has been more popular than the direct deposition process and the high temperature process. However, the laser thermal treatment process has problems that the phase transition is not uniform and the associated apparatuses of high price are required. In addition, since the productivity of the laser thermal treatment process is low, it is not suitable for manufacturing the polysilicon on a wide-area substrate.
The other method of crystallizing the amorphous material, particularly, the amorphous silicon is a solid-phase crystallization method capable of obtaining a uniform phase transition by using the associated apparatuses of low price. However, the method also has problems that the time of the crystallization is too long and the productivity is too low. In addition, since the temperature of the crystallization is high, a glass substrate can not be used.
On the other hand, a phase transition method of the amorphous material by using a metal has an advantage that the phase transition can be obtained at a low temperature in a short time in comparison with the solid-phase crystallization method. Therefore, many researches have been made on the method. The method includes a metal induced crystallization method.
In the metal-induced crystallization method, a specific kind of metal is directly contacted with at least one portion on the thin film of the amorphous material. And then, the phase transition is performed laterally from the contacted portion. Otherwise, the metal is doped into the thin film of the amorphous material to achieve the phase transition of the amorphous material. The examples are illustrated in FIGS. 1a to 1c. 
Now, the conventional metal-induced crystallization method will be described in detail. Firstly, a buffer layer 20 is formed on a dielectric substrate 10. And then, an amorphous material 30 is deposited on the buffer layer 20 by a chemical vapor deposition (CVD) method. Next, an oxide film is formed as a cap layer 40 (see FIG. 1a).
After the cap layer 40 is formed, the cap layer 40 is patterned by using a photolithography, so that a metal can be in contact with at least one portion of the amorphous material 30. The metal 50 is deposited on the patterned cap layer 40 and the amorphous material 30 which is exposed by the patterning process. Next, a thermal treatment is performed to partially grow grains 32 and 34 in the amorphous material, so that phase-transitioned thin films 32 and 34 can be obtained (see FIG. 1b).
However, in the conventional method of performing the phase transition on the amorphous material, the characteristics of the corresponding device are deteriorated due to the metal contamination on the region where the amorphous material 30 is directly contacted with the metal 50. Therefore, an additional process of removing the region is necessary, so that the productivity of the devices may be drastically lowered.
In addition, in the case where source and drain regions in a transistor are patterned to form a thin film or in the other case where one of the source and drain regions in the transistor are patterned to form the thin film, there is a problem that the amorphous material can not be completely phase-transitioned and the amorphous material region 37 remains (see FIG. 1c).
Namely, although the conventional metal-induced crystallization method of the amorphous material has an advantage of reducing the temperature of the crystallization, there is the problem that the intrinsic characteristics of the thin film are deteriorated by the contamination due to the metal which is permeated into the phase-transitioned thin film.
As a result, in order to use the metal-induced crystallization method of the amorphous material, it is necessary to minimize the metal contamination of the thin film. In order to minimize the contamination of the thin film, the most important thing is to reduce the amount of the metal used therein. The approaches disclosed to solve these problems include one method where metal ions having a concentration of 1012 to 1014 cm−2 are deposited by using an ion implantation apparatus and then a high temperature process, a rapid thermal treatment process, or a laser illumination process is performed. And, it also includes the other method where a viscous organic thin film and a liquid-phase metal are mixed in the convention metal-induced crystallization method, a deposition process is performed by a spin coating method, a thermal treatment process is performed, and then, the amorphous material is phase-transitioned.
However, the disclosed approaches have failed to effectively prevent the surface contamination of the thin film during crystallization. In addition, the approaches still have problems in terms of increasing the size of the grains and improving their uniformity